The clicking noise is a hunting sound -- cats make it when they see some kind of prey -- a sparrow hopping just too far away to be caught, or singing from a lower branch as if trying to tease the cat. It is made by the teeth clicking together, much like ours click when we are so cold our teeth "chatter." Essentially it means -- I want, I want, I want, oh, oh, I want that I want that.... Some cats do it and some don't. Among my own feline brood it often is done by a cat who has had the benefit of living with their mother and who has learned the rudiments of hunting their own food, and is less often done by a cat that I have bottle reared.